WASHINGTON — Teachers who aren’t up to snuff shouldn’t be in classrooms and should find new professions, the head of the 1.5 million-member American Federation of Teachers said Monday.

Randi Weingarten told a gathering of her union’s rank-and-file members that they should be more vigilant about their colleagues’ abilities and said weak educators who don’t make improvements only hurt the profession. The tough warning comes as state education chiefs have been trying to implement tougher standards for those in the classrooms and weed out teachers whose students aren’t making progress.

“If someone can’t teach, after they’ve been prepared and supported, they shouldn’t be in our profession,” Weingarten said to applause from more than 2,000 union members meeting here.

Weingarten said too often teachers are left “to sink or swim” without help from more senior teachers or their schools’ leaders. She stressed new teachers should be mentored and offered more training if their college coursework was inadequate.

But she acknowledged she did not want to be in the business of defending all teachers. She said union-backed evaluation systems would help keep successful teachers in the classrooms — and remove those who aren’t helping students.

“It recasts tenure as a guarantee of fairness and due process, not as an excuse for managers not to manage, and not to cloak incompetence,” Weingarten said.

School reform advocates often point to teacher tenure as a roadblock to change. Those advocates say the promise of essential lifetime jobs has left teachers unaccountable and leaves students in classrooms with uninspired teachers. If a teacher has no real prospect of being dismissed, there is little a school official can do to force changes.

Correct, Weingarten said. Some teachers fit that bill and should be kicked out of classrooms if they have been given a manageable classroom and sufficient supplemental training.

“We want people to be prepared and supported,” she told reporters after the speech. “But if they can’t do our job, which is the hardest job in America, then they shouldn’t be there.”

But she also had harsh words for those who would fire teachers based on students’ performances on standardized tests. That practice has gained popularity as states have implemented school improvement plans to hold teachers accountable.

“I have a plea for those who fixate on how to dismiss teachers — a plea — fixate instead on how we nurture, support and keep them,” Weingarten said.